A common criticism of plated metal contacts for photovoltaic devices is the poor adhesion to the silicon material. This problem is exacerbated when thermal cycling occurs due to the thermal expansion mismatch between the silicon and the metal and also when low metal/silicon interface area is used which is common in high performance cells due to the improved electrical performance. There is consequently often a trade-off between electrical performance and durability. The laser doped solar cell faces the same challenges [S. R. Wenham, M. A. Green “A Self-Aligning Method for Forming a Selective Emitter and Metallization in a Solar Cell”, International Patent No. PCT/AU/1999/00522, July, 1999], with the best electrical performance achieved when the laser doped region is flat and smooth which also corresponds to the lowest subsequent metal/silicon interface area after plating and therefore the poorest adhesion.
The laser doping process is most commonly carried out using a Q-switched 532 nm NdYAG laser. The laser beam is scanned across the surface of typically a silicon nitride (ARC) coated silicon wafer in the presence of a dopant source so that sufficient energy is delivered to the silicon to melt it and allow the dopants to diffuse into the molten silicon. This process simultaneously damages or removes the silicon nitride layer therefore exposing the recrystallised doped silicon surface so that subsequent metal plating, often Ni/Cu/Ag or Ni/Cu/Sn, can be effected. Even when a textured or roughened silicon surface is used, the laser melting/doping process, if done for sufficient duration to allow proper diffusion/mixing of the dopants, leads to the formation of a relatively smooth surface as shown in FIGS. 8(b) and 9. Subsequent metal adhesion to such surfaces, as shown in FIG. 10, is therefore a challenge.